The present invention relates to antennas for use in a wireless communications system and, more particularly, to a canister antenna for mitigating Passive InterModulation (PIM).
Typical cellular systems divide geographical areas into a plurality of adjoining cells, each cell including a wireless cell site or “base station.” The cell sites operate within a limited radio frequency band and, accordingly, carrier frequencies must be used efficiently to ensure sufficient user capacity in the system.
One way to increase call carrying capacity includes the creation of additional cells sites across a smaller geographic area. Partitioning the geographic area into smaller regions, however, involves purchasing additional equipment and real estate for each site. In addition to being fiscally inefficient, this approach can be aesthetically objectionable inasmuch as such cell sites are typically accompanied by unattractive cellular antennas and base station housing structures. Accordingly, this problem must be addressed by more economical and effective antennas that do not detract from the surrounding environment.
To improve the quality and reliability of wireless systems, service providers often rely on “antenna diversity”. Diversity improves the ability of an antenna to see an intended signal around natural geographic features of the landscape, including man-made structures such as high-rise buildings. A diversity antenna array helps to increase coverage as well as to overcome fading. Antenna polarization is another important consideration when choosing and installing an antenna. For example, polarization diversity combines pairs of antennas with orthogonal polarizations to improve base station uplink gain. Given the random orientation of a transmitting antenna, when one diversity-receiving antenna fades due to the receipt of a weak signal, the probability is high that the other diversity-receiving antenna will receive a strong signal. Most communications systems use vertical, slant or circular polarization.
“Beam shaping” is another method to optimize call carrying capacity by providing the most available carrier frequencies within demanding geographic sectors. Oftentimes user demographics change such that the base transceiver stations have insufficient capacity to deal with current local demand within a localized area. For example, a new housing development within a cell may increase demand within that specific area. Beam shaping can address this problem by distributing the traffic among the transceivers to increase coverage in the demanding geographic sector.
Prior art beam shaping solutions utilize complex beam-forming devices (LPAs, controllable phase shifters, etc.), many of which are not well-suited for deployment atop a masthead or tower-top. A significant design effort involves the use of 2- and 3-sector antennas optimized to provide beam-forming within a canister for purpose of increasing “long term evolution” (4G LTE) data rates in a small cellular network.
Additionally, cellular carriers often use GPS signals for accurate timing. In addition to longitude and latitude, the Global Positioning System (GPS) provides an accurate time source. Each GPS satellite contains multiple atomic clocks that integrate time data onto GPS signals to calculate a geographic position. The time data, in addition to calculating longitude and latitude, allows receivers to determine time within a billionth of a second. Cellular carriers use this precise time data for synchronization and operational efficiency of their networks. The GPS time data keeps the base stations in perfect synchronization which, in turn, allows mobile handsets to share the limited radio frequency spectrum efficiently. It is well-known to add a GPS antenna onto a base station for this purpose, however, prior art GPS solutions utilize separate antennas and separate coaxial cables from antenna to receiver. As such, this arrangement is complex and expensive.
The foregoing background describes some, but not necessarily all, of the problems, disadvantages and shortcomings related to canister antennas.